Enhanced stripping of low-K films using downstream gas mixing

ABSTRACT

The present invention pertains to methods for removing unwanted material from a work piece. More specifically, the invention pertains to stripping photo-resist material and removing etch-related residues from a semiconductor wafer during semiconductor manufacturing. Methods involve implementing a hydrogen plasma operation with downstream mixing with an inert gas. The invention is effective at stripping photo-resist and removing residues from low-k dielectric material used in Damascene devices.

BACKGROUND

The present invention pertains to methods and systems for strippingphoto-resist material and removing etch-related residues from thesurface of a partially fabricated integrated circuit in preparation forfurther processing. More specifically, the invention pertains to methodsand systems for implementing a plasma operation that includesintroducing an inert gas downstream of the plasma source. The inventionis effective at efficiently stripping photo-resist and removing residuesfrom low-k dielectric layers after etching processes used to produceDamascene devices.

Damascene processing techniques are often preferred methods in modernintegrated circuit manufacturing schemes because they require fewerprocessing steps and offers a higher yield than other methods. Damasceneprocessing involves forming metal conductors on integrated circuits byforming inlaid metal lines in trenches and vias in a dielectric layer(inter-metal dielectric). As part of the Damascene process, a layer ofphotoresist is deposited on a dielectric layer. The photoresist is alight-sensitive organic polymer which can be “spun on” in liquid formand dries to a solid thin film. The photosensitive photoresist is thenpatterned using light through the mask and wet solvent. A plasma etchingprocess (dry etch) is then used to etch exposed portions of dielectricand transfer the pattern into the dielectric, forming vias and trenchesin the dielectric layer.

Once the dielectric layer is etched, the photoresist must be strippedand any etch-related residues must be thoroughly removed beforesubsequent processing to avoid embedding impurities in the device.Conventional processes for stripping photoresist employ a plasma formedfrom a mixture of gases with the presence of oxygen in the plasma. Thehighly reactive oxygen based plasma reacts with and oxidizes the organicphotoresist to form volatile components that are carried away from thewafer surface.

Highly oxidizing conditions are also generally unsuitable for use on lowdielectric constant (low-k) materials. Low-k materials have been used asinter-metal and/or inter-layer dielectrics between conductiveinterconnects employed to reduce the delay in signal propagation due tocapacitive effects. The lower the dielectric constant of the dielectricmaterial, the lower the capacitance of the dielectric and the lower theRC delay of the integrated circuit. Typically, low-k dielectrics aresilicon-oxide based materials with some amount of incorporated carbon,commonly referred to as carbon doped oxide (CDO). It is believed,although not necessarily proven, that the oxygen scavenges or removescarbon from the low-k materials. In many of these materials such asCDOs, the presence of carbon is instrumental in providing a lowdielectric constant. Hence, to the extent that the oxygen removes carbonfrom these materials, it effectively increases the dielectric constant.As processes used to fabricate integrated circuits move toward smallerand smaller dimensions and requires the use of dielectric materialshaving lower and lower dielectric constants, it has been found that theconventional strip plasma conditions are not suitable.

Hydrogen plasmas or hydrogen-based plasmas with a weak oxidizing agentare effective at stripping photo-resist and removing residues from low-kdielectric layers without the problems associated with conventionalstrip plasmas. However, these methods require a high hydrogen flow toachieve an acceptable strip rate. Because high hydrogen flow requirescostly abatement and pump systems, it is desirable to have hydrogen flowas low as possible while maintaining an acceptable strip rate. Inaddition, it is desirable to reduce hydrogen flow due to hydrogen'sflammability and the dangers associated with handling and abating it.

Others have reported using hydrogen-based plasmas with inert gases suchas hydrogen and helium introduced with hydrogen at the plasma source.Han et al (U.S. Pat. Nos. 6,281,135 and 6,638,875) describe using amixture of hydrogen, helium and fluorine and Zhao et al (U.S. Pat. Nos.5,660,682 and 6,204,192) describe using a mixture of hydrogen and argon.However, helium or argon ions in the plasma have harmful effects.Mixtures of hydrogen and helium result in high plasma damage on low-kmaterials due to the long life of ionized helium plasma. Ionized argoncauses unwanted sputtering of the quartz material in the plasma tube(the portion of some reactors where the plasma is formed). Introductionof argon to hydrogen plasmas has also been shown to reduce strip rate.

What is needed therefore are improved and methods and apparatus forstripping photoresist and etch-related materials from dielectricmaterials, especially from low-k dielectric materials, which reduce therequired hydrogen flow rate while maintaining an acceptable strip rate.

SUMMARY OF THE INVENTION

The present invention addresses the aforementioned need by providingimproved methods and an apparatus for stripping photoresist and removingetch-related residues from dielectric materials. An inert gas isintroduced to the plasma downstream of the plasma source and upstream ofa showerhead that directs gas into the reaction chamber. The inert gasmixes with the plasma, reducing the required hydrogen flow rate andimproving strip rate and strip rate uniformity.

In one aspect of the invention, methods involve removing material from awork piece in a process chamber according to the following operations:(a) introducing a gas comprising hydrogen into a plasma source, (b)generating a plasma from the gas introduced into the plasma source, (c)introducing an inert gas downstream of the plasma source and upstream ofthe work piece; and (d) removing the material from the work piece.Another aspect of this invention relates to an apparatus for removingmaterial from a work piece surface comprising: (a) a plasma source, (b)a gas inlet for introducing a hydrogen-based gas into the plasma source,(c) a gas inlet for introducing an inert gas downstream of the plasmasource and upstream of the work piece; and (e) a process chamber.

The methods and apparatus of the invention may be used to removephotoresist/etch byproduct materials from dielectric materials on apartially fabricated integrated circuit. In a preferred embodiment, thework piece comprises a single or dual Damascene device.

In preferred embodiments of the invention, the inert gas comprises argonor helium. In a particularly preferred embodiment, the inert gascomprises argon. In preferred embodiments, the inert gas flow rate isbetween 0.15 and 10.0 times the hydrogen flow rate. In particularlypreferred embodiments, the inert gas flow rate is between 0.75 and 6.0times the hydrogen flow rate.

The inert gas is introduced downstream of the plasma source and upstreamof work piece via gas inlets. In a preferred embodiment, the inert gasis introduced upstream of a showerhead that directs the plasma/inert gasmixture into the process chamber.

In preferred embodiments, the gas inlets comprise jets which may bepositioned to optimize mixing of the inert gas with the plasma. Inpreferred embodiments, the jets are positioned such that the inert gasenters at a zero degree angle from the bottom of the plasma source.

In preferred embodiments of the invention, the gas comprising hydrogenintroduced into the plasma source further comprises a weak oxidizingagent. In a particularly preferred embodiment, the weak oxidizing agentcomprises carbon dioxide.

The plasma source used in accordance with the methods and apparatus ofthe invention may be any type of plasma source. In a preferredembodiment an RF plasma source is used.

The process chamber used in accordance with the methods and apparatus ofthe invention may be any suitable process chamber. The process chambermay be one chamber of a multi-chambered apparatus or it may be part of asingle chamber apparatus.

These and other features and advantages of the present invention will bedescribed in more detail below with reference to the associateddrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration showing an apparatus according to oneembodiment of the claimed invention and suitable for practicing themethods of the claimed invention.

FIG. 2 is a graph showing the effect of downstream mixing with argon orhelium flows on dry etch/photoresist strip rate of a wafer and theuniformity of the strip rate over the wafer.

FIG. 3 is a graph showing the effect of downstream mixing with argon onchange in k value of a low-k dielectric.

FIGS. 4 a-4 c are plots representing strip rate topography across thesurfaces of 3 wafers treated at different conditions with 3-jetdownstream argon gas inlet.

FIG. 5 is a chart showing argon gas inlet jet angle on strip rate andstrip rate uniformity of a wafer in a process in accordance with thisinvention.

FIGS. 6 a and 6 b are charts showing the effect of inert gas inlet jetangle on strip rate and strip rate uniformity of a wafer in a process inaccordance with this invention.

FIG. 7 is a graph showing the effect of hydrogen flow rate on dryetch/photoresist strip rate of a wafer and strip rate uniformity overthe wafer.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Introduction

In the following detailed description of the present invention, numerousspecific embodiments are set forth in order to provide a thoroughunderstanding of the invention. However, as will be apparent to thoseskilled in the art, the present invention may be practiced without thesespecific details or by using alternate elements or processes. In otherinstances well-known processes, procedures and components have not beendescribed in detail so as not to unnecessarily obscure aspects of thepresent invention.

In this application, the terms “semiconductor wafer”, “wafer” and“partially fabricated integrated circuit” will be used interchangeably.One skilled in the art would understand that the term “partiallyfabricated integrated circuit” can refer to a silicon wafer during anyof many stages of integrated circuit fabrication thereon. The followingdetailed description assumes the invention is implemented on a wafer.However, the invention is not so limited. The work piece may be ofvarious shapes, sizes, and materials. In addition to semiconductorwafers, other work pieces that may take advantage of this inventioninclude various articles such as printed circuit boards and the like.

As mentioned previously, the methods and apparatus of the invention maybe used to efficiently and effectively to remove materials from a low-kdielectric materials. The invention is not limited to dielectricmaterials or low-k dielectric materials. The invention is also notlimited to any particular category of low-k dielectrics. For instance,described methods and apparatus may be effectively used with dielectricswith k values less than 4.0 (“first generation” low-k dielectrics),dielectrics with k values less than about 2.8 (“second generation” low-kdielectrics) and dielectrics with k values less than about 2.0(“ultra-low-k” dielectrics). The low-k dielectric may be porous ornon-porous (sometimes referred to as a “dense” low-k dielectric).Generally, low-k dense dielectrics are those having k values no greaterthan 2.8 and low-k porous dielectrics are those having k values nogreater than 2.2. Low-k dielectrics of any suitable composition may beused, including silicon oxide based dielectrics doped with fluorineand/or carbon. Non-silicon oxide based dielectrics, such as polymericmaterials, may also be used. Any suitable process may be used to depositthe low-k dielectric, including as spin-on deposit and CVD deposittechniques. In the case of forming porous dielectrics, any suitablemethod may be used. A typical method involves co-depositing asilicon-based backbone and an organic porogen and subsequently removingthe porogen component, leaving a porous dielectric film. Other methodsinclude sol-gel techniques. Specific examples of suitable low-k filmsare carbon based spin-on type films such as SILK™ and CVD depositedporous films such as Coral™.

The methods and apparatus of the invention use plasmas that are producedfrom gases that contain hydrogen. The gases may also contain a weakoxidizing agent. One skilled in the art will recognize that the actualspecies present in the plasma may be a mixture of different ions andmolecules derived from the hydrogen and/or weak oxidizing agent. It isnoted that other species may be present in the reaction chamber, such assmall hydrocarbons, carbon dioxide, water vapor and other volatilecomponents as the plasma reacts with and breaks down the organicphotoresist and other residues. One of skill in the art will alsorecognize that reference to the initial gas/gases introduced into theplasma is/are different from other gas/gases that may exist after theplasma is formed.

FIG. 1 is a schematic illustration of an apparatus 100 according to oneembodiment of the claimed invention. The apparatus depicted in FIG. 1 isalso suitable to practice methods of claimed invention. Apparatus 100has a plasma source 101 and a process chamber 103 separated by ashowerhead assembly 105. Plasma source 101 is connected to gas inlet111. Showerhead 109 forms the bottom of showerhead assembly 105. Inertgas inlets 113 are downstream of plasma source 101 and upstream of wafer115 and showerhead 109. Inside process chamber 103, a wafer 115 withphotoresist/dry etch byproduct material rests on a platen (or stage)117. Platen 117 may be fitted with a heating/cooling element. In someembodiments, platen 117 is also configured for applying a bias to wafer115. Low pressure is attained in reaction chamber 103 via vacuum pumpand conduit 119.

In operation, a gas is introduced via gas inlet 111 to the plasma source101. The gas introduced to the plasma source contains the chemicallyactive species that will be ionized in the plasma source to form aplasma. Gas inlet 111 may be any type of gas inlet and may includemultiple ports or jets. Plasma source 101 is where the active species ofthe gas introduced to the source is generated to form a plasma. In FIG.1, an RF plasma source is shown with induction coils 115. Inductioncoils 115 are energized and the plasma is generated. An inert gas isintroduced via gas inlets 113 upstream of the showerhead and downstreamof the plasma source. The inert gas mixes with the plasma. Gas inlets113 may be any type of gas inlets and may include multiple ports or jetsto optimize mixing the inert gas with the plasma. Showerhead 109 directsthe plasma/inert gas mixture into process chamber 103 through showerheadholes 121. There may be any number and arrangement of showerhead holes121 to maximize uniformity of the plasma/gas mixture in process chamber103. Showerhead assembly 105, which has an applied voltage, terminatesthe flow of some ions and allows the flow of neutral species intoprocess chamber 103. As mentioned, wafer 115 may be temperaturecontrolled and/or a RF bias may be applied. The plasma/inert gas mixtureremoves the photoresist/etch byproduct material from the wafer.

In some embodiments of the claimed invention, the apparatus does notinclude showerhead assembly 105 and showerhead 109. In theseembodiments, the inert gas inlets 113 introduce the inert gas directlyinto the process chamber where it mixes with the plasma upstream ofwafer 115.

FIG. 2 is graph showing the effects of downstream mixing with argon andhelium on the etch/photoresist strip rate of a wafer and the uniformityof the strip rate over the wafer for various hydrogen flows.

Hydrogen flow rate is shown on the x-axis. Net strip rate is shown onthe left y-axis in Å/min. Net strip rate does not include any shrinkagedue to evaporation of the solvent in the photoresist. Strip rateuniformity over the wafer, calculated as 1 standard deviation/averagestrip rate, is shown on the right y-axis. FIG. 2 shows strip rate ishighest when the hydrogen based plasma is mixed downstream with argon.In particular, mixing 3.5 slm hydrogen/30 sccm carbon dioxide with 3 slmargon resulted in a higher strip rate than achieved with 6.5 slmhydrogen/30 sccm carbon dioxide and no mixing. Thus, downstream mixingresults in a strip rate superior to that obtained with a conventionalprocess and a 44% reduction in hydrogen flow.

Strip rates when the hydrogen-based plasma is mixed downstream withhelium are also greater than when there is no downstream mixing.Further, strip rate uniformity is shown to be improved for most caseswith downstream mixing. Thus, FIG. 2 demonstrates that strip rate andstrip rate uniformity are maintained for lower hydrogen flows usingmethods and apparatus in accordance with the present invention.

As discussed above, many conventional photoresist/etch strip processesare not effective to strip low-k dielectric materials because theyeffectively raise the dielectric constant. FIG. 3 is chart showing theeffect of various flow rates of argon introduced downstream of a plasmasource on change in dielectric constant of a low-k dielectric. A 300 mmwafer with 200 mm Novellus Coral low-k film was stripped at 1 Torr. Flowrate of hydrogen plus inert gas was kept constant at 6.5 slm. 30 sccm ofcarbon dioxide was introduced with hydrogen to the plasma source. A 1300W RF plasma source was used. The reference value shown is the Δk for awafer not exposed to plasma and reflects the Δk due to exposure toambient conditions.

Typically, a Δk of less than 0.1 is acceptable. The reference value inFIG. 3 shows the Δk resulting when the wafer is exposed to air only.FIG. 3 shows that Δk values for downstream mixing with argon are allless than or about the reference value. All are well below maximumacceptable Δk. Thus, FIG. 3 shows that the methods and apparatus of theinvention are effective to strip low-k dielectric materials.

Process Parameters Upstream Inlet Gas

A hydrogen-based gas is introduced to the plasma source. Typically thegas introduced to the plasma source contains the chemically activespecies that will be ionized in the plasma source to form a plasma. Inpreferred embodiments, the gas introduced to the plasma source furthercomprises a weak oxidizing agent such as carbon dioxide, carbonmonoxide, nitrogen dioxide, nitrogen oxide and water. In particularlypreferred embodiments, the weak oxidizing agent is carbon dioxide. Inparticularly preferred embodiments, the gas introduced to the plasmasource comprises between about 0.1% to about 1.0% carbon dioxide byvolume. Applicants disclose methods of stripping photoresist and etchmaterials from a low-k dielectric using hydrogen-based plasmas with weakoxidizing agents in previously filed U.S. patent application Ser. No.10/890,653, which is hereby incorporated by reference. The gasintroduced to the plasma source may further comprise other gases asneeded, for example, to remove any plasma residue from the wafer. In apreferred embodiment, a small amount of nitrogen triflouride isintroduced at the last station (in a multi-station process) to removeresidue from the wafer.

Plasma Generation

Any known plasma source may be used in accordance with the invention,including a RF, DC, microwave any other known plasma source. In apreferred embodiment, a downstream RF plasma source is used. Typically,the RF plasma power for a 300 mm wafer ranges between about 300 Watts toabout 3 Kilowatts. In a preferred embodiment, the RF plasma power isbetween about 1000 Watts and 1500 Watts.

Inert Gas

Any inert gas may be introduced downstream of the plasma source andupstream of the showerhead for mixing with the plasma. In a preferredembodiment, the inert gas is argon or helium. In a particularlypreferred embodiment, the inert gas is argon. However, any inert gas,such as nitrogen, may be used. In preferred embodiments, the inert gasflow rate is between about 0.15 and 10.0 times the hydrogen flow rate.In particularly preferred embodiments, the inert gas flow rate isbetween about 0.75 and 6.0 times the hydrogen flow rate.

Inert Gas Inlet

The inert gas inlet may be any type of gas inlets and may includemultiple ports or jets to maximize mixing with the plasma. The angle ofthe inlet jets may also optimized to maximize mixing. In a preferredembodiment, there are at least four inert gas inlet jets. In aparticularly preferred embodiment, there are sixteen inlet jets. In apreferred embodiment the angle of the inlet jets, as measured from thebottom of the plasma source, is zero degrees so that the inert gas isinjected perpendicular to the direction of flow of the plasma enteringthe showerhead assembly (or the process chamber if there is noshowerhead assembly) from the plasma source. An angle of zero degreesalso corresponds a direction parallel to the face of the work piece.

FIGS. 4 a-4 c are plots representing strip rate topography across thesurfaces of 3 wafers treated at different conditions with 3-jetdownstream argon gas inlet. FIG. 4 a shows the topography of a waferexposed to a plasma with no downstream mixing. FIG. 4 b shows thetopography of a wafer exposed to a plasma mixed with 1 slm argondownstream, and FIG. 4 c shows topography of a wafer exposed to a plasmamixed with 3 slm argon downstream. Total hydrogen plus argon flow ratewas 6.5 slm for all figures. 30 sccm carbon dioxide was also used.Temperature and pressure were kept at 280° C. and 1 Torr and exposuretime at 60 seconds.

Areas of higher strip rate 401 can be seen in FIGS. 4 a and 4 b. Thisindicates that more than three inert gas inlet jets should be used toachieve better mixing and strip rate uniformity.

FIG. 5 is a chart showing argon gas inlet jet angle on strip rate andstrip rate uniformity of a wafer in a process in accordance with thisinvention. 1.2 μm of a photoresist was deposited on the dielectric. Onestation was used. 60 seconds of stabilization time to pre-heat the waferbefore exposing it to plasma was used followed by 60 seconds of exposureto the plasma. Hydrogen/carbon dioxide flow rates were 3 slm/30 sccm.Downstream argon flow rate was 5 slm.

Net strip rate is shown on the left y-axis in Å/min. Strip rateuniformity over the wafer, calculated as 1 standard deviation/averagestrip rate, is shown on the right y-axis. Strip rate was maximized whenthe argon inlet jets were at zero degrees. No difference in strip rateuniformity was detected.

FIGS. 6 a and 6 b also show that a jet angle of zero degrees maximizesstrip rates. FIGS. 6 a and 6 b show the results of models that predicthelium mass fraction as a function of wafer radius for helium injectedat −45°, 0° and 45°. Helium mass fraction is proportional to strip rate.Results shown FIG. 6 a were found for flow 5.5 slm hydrogen, 1 slmhelium and in FIG. 6 b for 1 slm hydrogen, 5.5 slm helium. The chartsshow that for both cases, strip rate is maximizes for an inlet jet angleof zero degrees.

Showerhead Assembly

Preferred embodiments of the present invention include a showerheadassembly. The showerhead assembly may have an applied voltage,terminates the flow of some ions and allows the flow of neutral speciesinto the reaction chamber. The assembly includes the showerhead itselfwhich may be a plate having holes to direct the plasma and inert gasmixture into the reaction chamber. The showerhead redistributes theactive hydrogen from the plasma source over a larger area, allowing asmaller plasma source to be used. The number and arrangement of theshowerhead holes may be set to optimize strip rate and strip rateuniformity. Fewer holes improve uniformity, but increase recombinationof the plasma ions and electrons which results in a lower strip rate. Ifthe plasma source is centrally located over the wafer, the showerheadholes are preferably smaller and fewer in the center of the showerheadin order to push the active gases toward the outer regions. Theshowerhead preferably has at least 100 holes.

In embodiments in which there is no showerhead assembly, the plasmaenters the process chamber directly.

Process Chamber

The process chamber may be any suitable reaction chamber. It may be onechamber of a multi-chambered apparatus or it may simply be a singlechamber apparatus. The chamber may also include multiple stations wheredifferent wafers are processed simultaneously. The process chamber maybe the same chamber where the etch takes place or a different chamberthan where the etch takes place. Process chamber pressure may range from300 mTorr to 2 Torr. Preferably the pressure ranges from 0.9 Torr to 1.1Torr.

Work Piece

In preferred embodiments, the work piece used in accordance with themethods and apparatus of the invention is a semiconductor wafer. Anysize wafer may be used. Most modern wafer fabrication facilities useeither 200 mm or 300 mm wafers. Process conditions may vary dependingupon the wafer size. In particularly preferred embodiments, the workpiece comprises a single or dual Damascene device.

In some embodiments of the invention, it is desired to keep the workpiece at a particular temperature during the application of plasmas toits surface. Preferred wafer temperatures can range between about 220degrees and about 400 degrees Celsius.

In preferred embodiments, the surface of the work piece comprises low-kdielectric materials, including carbon-doped low-k dielectric materialssuch as carbon-doped oxides (CDOs). Non-porous and porous dielectricmaterials, including CDOs and other compositions may be used.

EXAMPLES

300 mm sized wafers were processed (i.e., photoresist stripped) on astrip station. Each wafer was covered with 1.2 μm of photoresist. RFpower was set at 1300 W and pressure at 1 Torr. 30 sccm of carbondioxide was introduced into the plasma source with the hydrogen. Flowrate of hydrogen plus inert gas was kept at 6.5 slm. The results areshown in FIG. 2 as described above.

300 mm sized wafers were processed. Each wafer was covered with 1.2 μmof photoresist. RF power was set at 1500 W and pressure at 1 Torr. Argonflow rate was kept at 6 slm. Net strip rate and strip rate uniformitywas found for argon/hydrogen rations of hydrogen flow rates of 1 slim,1.5 slm, 2.0 slm, 2.5 slm and 3.0 slm (i.e. for argon/hydrogen ratios of6.0, 4.0, 3.0, 2.4 and 2.0). Results are shown in FIG. 7. All examplesresulted in net strip rates greater than 3000 Å/min and strip rateuniformities of less than 4%.

Seven 300 mm wafers were processed on a five-station chamber with RFpower set at 1300 W and pressure at 1.1 Torr. Total hydrogen flow ratewas kept at 15 slm and total carbon dioxide flow rate was kept at 150sccm. Total argon flow rate was kept at 30 slm. The average net striprate of the seven wafers was 2951 Å/min. Strip rate uniformity wascalculated for six of the wafers with the average found to be 3.61%.

Seven 300 mm wafers were processed with RF power set at 1200 W andpressure at 0.9 Torr. Total hydrogen flow rate was kept at 12 slm andtotal carbon dioxide flow rate was kept at 150 sccm. Total argon flowrate was kept at 24 slm. The average net strip rate of the seven waferswas 2807 Å/min. Strip rate uniformity was calculated for six of thewafers with the average found to be 3.00%.

Additional experimental results are shown in Table 1 which shows striprates and strip rate uniformity obtained for various argon flow rates,pressures, and RF powers. All data was collected using in a five-stationchamber with hydrogen flow of 3 slm per station (15 slm total) andcarbon dioxide flow of 30 sccm per station (150 sccm total).

TABLE 1 Experimental results of downstream mixing with argon Downstreamargon flow rate per station RF Pressure Net strip Rate Strip rate (slm)power (W) (Torr) (Å/min) uniformity (%) 4 1300 1.0 2921 4.4 4 1300 1.12707 3.3 4 1500 1.0 2669 4.6 4 1500 1.1 2943 4.3 6 1300 1.0 3298 3.5 61300 1.1 2718 3.0 6 1500 1.0 3046 3.3 6 1500 1.1 2953 3.6The target strip rate for the examples shown in Table 1 was 2200 Å/minwith a uniformity of less than 4%. All of the above examples meet thetarget strip rate and most meet the target uniformity.

Note that experimental results for these specific examples are shown toclarify and illustrate the effectiveness of methods of the invention andare not meant to limit the invention to any particular embodiments.

1-21. (canceled)
 22. An apparatus for removing material from a workpiece surface in a reaction chamber comprising: a plasma source; a gasinlet for introducing a hydrogen-based gas into the plasma source; a gasinlet for introducing an inert gas downstream of the plasma source andupstream of the work piece; and a process chamber.
 23. The apparatus ofclaim 22 further comprising a showerhead assembly, said assemblycomprising a showerhead for directing the plasma and inert gas into theprocess chamber.
 24. The apparatus of claim 23 wherein the showerheadcomprises at least 1000 holes.
 25. The apparatus of claim 23 furthercomprising a platen for supporting the work piece.
 26. The apparatus ofclaim 23 further comprising an RF coil for generating plasma in theplasma source.
 27. The apparatus of claim 23 wherein the inert gas inletcomprises at least four inlet jets.
 28. The apparatus of claim 27wherein the gas jets are a at zero degree angle from the bottom of theplasma source.
 29. The apparatus of claim 27 wherein the inert gas inletcomprises at least sixteen inlet jets.
 30. The apparatus of claim 22wherein the inert gas inlet is configured to inlet gas parallel to theface of the work piece.
 31. The apparatus of claim 22 wherein the inertgas inlet comprises a plurality of inlet jets, wherein the angle of theinlet jets as measured from the bottom of the plasma source is zerodegrees